Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. Oil sand deposits are typically extracted by surface mining. The mined oil sand is trucked to crushing stations for size reduction, and fed into slurry preparation units such as tumblers, cyclofeeders, and the like where hot water and, optionally, caustic are added to form a slurry for bitumen separation. The oil sand slurry may be screened through a screening device, where additional hot water may be added to clean the rejects prior to delivery to a rejects pile. The (screened) oil sand slurry is collected in a vessel and then pumped through a hydrotransport pipeline designed to condition and carry oil sand slurry from mining to extraction facilities to ensure sufficient conditioning of the oil sand slurry. During the conditioning stage in the hydrotransport pipeline, the aeration of slurry occurred where bitumen is attached to air bubbles, creating a lower density bitumen-air aggregates.
The conditioned slurry is then fed to a primary separation vessel (“PSV”). In the PSV, the slurry is allowed to separate under quiescent conditions for a prescribed retention period into a top layer of bitumen froth, a middle layer of middlings (i.e., warm water, fines, residual bitumen), and a bottom layer of coarse tailings (i.e., warm water, coarse solids, residual bitumen).
The interface between the bitumen froth and middlings is well defined when processing ores which are relatively high in bitumen content and low in fines content. “Fines” are particles such as fine quartz and other heavy minerals, colloidal clay or silt generally having any dimension less than about 44 μM. “Good ores” are oil sand ores having high bitumen content (10-12%) and relatively low fines content (less than about 20%). In contrast, “poor ores” are oil sand ores having low bitumen content (7-10%) and relatively high fines content (greater than 30%).
Poor ores typically do not segregate properly. The problem of “sludging” in the PSV is triggered by high fines content, and is characterized by the deterioration of the interface between the bitumen froth and middlings due to an increase in the density of the middlings. Coarser mineral particles and bitumen become entrapped in the process slurry as a result of non-segregating settling. Such conditions result in lower bitumen recovery and poorer quality of bitumen froth, leading to a decrease bitumen production capacity through the froth treatment plant. Attempts to alleviate this problem include manipulating operation variables such as, for example, total water, caustic dosage, ore blending, and throughput. A further problem encountered with bitumen froth quality is low froth temperature as a result of reducing the bulk processing temperature. Hence, this may lead to production capacity restrictions in downstream froth heating equipment.